Resistance training improves whole muscle performance in inactive older adults. However, the intrinsic muscle functional adaptations underlying these improvements are not well-understood, nor is it clear that men and women derive the same functional benefits.
PURPOSE: We measured the skeletal muscle functional response at the whole body, tissue, cellular and molecular levels in inactive older men (M: n=7, 69±2 yrs) and women (W: n=10, 70±2 yrs) to a 14 week moderate-intensity resistance training program.
METHODS: Whole muscle performance (one repetition maximum, isometric and isokinetic knee extensor torque) and size (computed tomography), cellular contractile properties (force-velocity curves) and myosin-actin cross-bridge mechanics and kinetics (sinusoidal analysis) were measured pre- and post-training.
RESULTS: Resistance training increased whole muscle size (6-7%, p<0.05), one repetition maximum (48-71%, p<0.001) and isometric torque (6-15%, p<0.01) similarly in men and women, while isokinetic function was unchanged. In myosin heavy chain (MHC) I fibers, isometric tension (force per cross-sectional area) was increased in men and decreased in women with training (M: 7%, W: -9%, p=0.05), causing power output to be greater in men and lower in women at higher tensions (M: 12-44%, W: -28% to -61%). The differences in cellular function were explained at the molecular level by alterations in the number of strongly bound myosin heads (M: 21%, W: -9%, p<0.05) which, in turn, were due to sex differences in changes in myosin attachment time (M: 13%, W: -2%, p=0.06). In contrast to MHC I fibers, isometric tension was decreased in men and increased in women with training (M: -11%, F: 6%, p=0.06) in MHC IIA fibers and these differences were explained by increases in myofilament force transmissibility in women compared to men (M: -12%, W: 6%, p<0.05).
CONCLUSION: Resistance training improves whole muscle function in inactive older men and women, but there are sex differences in their fiber type response and the fundamental molecular adaptations that bring about the cellular phenotypes. These results indicate that exercise prescriptions may need to be sex-specific to maximize cellular and molecular performance, leading to optimal whole muscle function.
Supported by NIH Grants AG-033547 and AG-031303.